Page 288 - Handbook of Adhesives and Sealants

254 Chapter Seven

Interphase
region affected
by primer or
adhesion
promoters.

Primers Adherends

Adhesive

Figure 7.1 Primers and adhesion promoters provide a stronger interphase region hav-
ing improved adhesion and permanence.

molecular) layer between the substrate and the adhesive. Usually
chemical bonds are formed between the adhesion promoter and the
adhesive, and between the adhesion promoter and the substrate sur-
face. These bonds are stronger than the internal chemical bonds
within the adhesive. These new bonds also provide an interface region
that is more resistant to chemical attack from the environment. Ad-
hesion promoters are also often referred to as coupling agents.
Adhesion promoters can be applied by either incorporating them
directly into the adhesive formulation or by applying them to a sub-
strate, similar to a primer. When applied ‘‘in-situ’’, through the ad-
hesive formulation, the adhesion promoter migrates to the interface
region and attaches itself between the adhesive molecule and the sub-
strate before the adhesive cures.
Adhesion promoters or coupling agents are also used in applications
other than conventional adhesives or sealants. They give plastic com-
pounders a way of effectively improving properties and reducing the
overall cost of the compound. Adhesion promoters can be applied to
particulate fillers for reinforcing polymeric molding compounds and to
fibers for reinforcing composites. Thus, when discussing adhesion pro-
moters, the term ‘‘substrate’’ takes on the added possibilities of fillers,
reinforcements, etc. as well as conventional adherends.
Specific adhesion promoters have been developed for bonding glass
fibers to polyester resin, carbon fibers to epoxy resins, talc particles to
nylon, and reinforcing tire cord to rubber tire compounds. In these
applications, adhesion promoters not only improve the bond strength
and permanence of the interface but also increase the physical prop-
erties of the resulting bulk material. Bulk properties, such as tensile